Garden Allies: Ichneumonid Wasps

Ichneumonid (ick – new – mon – id) wasps are harmless hymenopterans, nonaggressive garden residents, and enormously beneficial garden allies—yet few know them by name. Gardeners encountering ichneumonids will find them walking about on plants with a characteristic staccato movement. With their long antennae in constant motion, they go in search of a suitable host insect in which to lay an egg. Ophion species adults, fluttering around lights in the evening, are familiar to both gardeners and non-gardeners. While many species may visit gardens, few are truly common; many species can only be seen in their wild habitat.

Western giant ichneumon (Megarhyssa nortoni)

Western giant ichneumon (Megarhyssa nortoni) Illus: Craig Latker

Slender, thread-waisted, and often with laterally compressed bodies, ichneumonids vary in size but most are small. All are parasitoids—parasitic insects that invariably kill their hosts—and are closely related to braconid wasps (see Pacific Horticulture April 2008). There are more parasitoids among the hymenoptera than any other order, with thousands of species in over 40 families. Many have fascinating life histories.

Ichneumonids are usually endoparasites, developing inside the host. Almost always, a single larva parasitizes a single host, although in some cases they are gregarious with multiple larvae in a single host. Most parasitoid species are host-specific, targeting one or a few closely related species of arthropods. Ichneumonids are further distinguished as habitat specialists and are very particular about where they search for hosts, seeking out leaf rolling or wood boring insects as hosts and not specializing in particular genera or species.

As reflected in the opening lines of verse, opposite, parasitoids are sometimes themselves parasitized. These secondary parasites are known as hyperparasitiods, a term first coined by Jean-Henri Fabre, a French entomologist who closely observed insect behavior and discovered this extraordinary biological relationship. These secondary parasites can reduce the beneficial effect of a primary parasitoid. But in our gardens, as in the wild, it is all just part of the web of life. Ichneumonids parasitize a wide variety of insects that undergo complete metamorphosis including flies, larvae of wood-boring beetles, weevil grubs, leaf miners, aphids, and even other hymenoptera—including other ichneumonid wasps.

Labeling insects as “beneficial” or “pest” is an oversimplification of complex relationships between organisms in nature. While ichneumonids parasitize an enormous number of pests, they also attack many beautiful and desirable butterfly and moth species, and prey on beneficial spider egg cases. The food web is much like a spider web: pluck one strand, and the entire structure is affected. Without aphids, I ask children on school visits, what would happen to ladybugs? A world without ladybugs is unimaginable, of course, and my question once caused a kindergartener to burst into tears when he realized the dire consequences of my question. We also rely on many species of less familiar allies in controlling populations of herbivorous insects in our gardens. In the end, it is more useful to think of insects in their ecological roles: herbivore, predator, parasite, and so on.

The parasitic wasp Hyposoter exiguae.

The parasitic wasp Hyposoter exiguae. Illus: Craig Latker

Many larval herbivorous insects in the garden are likely parasitized even though they appear healthy to the casual observer. While some parasitoids pupate externally, many complete their life cycle within the host, emerging only once they reach their adult stage. Ichneumonids parasitize insect hosts in larval and pupal stages. The Ophion species commonly found in gardens, for instance, parasitizes caterpillars including many moth species. Many useful ichneumonids attack sawflies and other agricultural pests. Although their ovipositors appear dangerous, ichneumonids generally do not sting. The formidable appearance of Megarhyssa species is only a terror to its prey, as they use their long ovipositor to drill into trees and deposit their eggs, where their larvae attack wood wasps and beetle larvae.

The complicated life history of many ichneumonids is yet unknown, and provides an opportunity for the enterprising naturalist to make discoveries. More than one butterfly and moth enthusiast, raising a caterpillar or collected pupa, has been surprised to find a wasp emerging from the cocoon, rather than the expected lepidoptera. Whatever emerges, it is sure to be fascinating to the garden entomologist!